1. Register a self-defined autoload function using the spl_autoload_register function
2. When a new non-existent class is in the code, the above function is called and the non-existent class name is passed into the function as a parameter.
3. Compatible with two ways, namespace corresponds to directory app test, class name underline divides corresponding directory app_test, all of which are test.php files under the corresponding app directory, and the class name should be the same as the file name.
4.set_include_path(), which can have multiple colons: separate, dynamically set the include_path configuration options in php.ini

<?php
class Application {
    private static $instance    = null;
    private $libPath            = './';
    private $phpExt             = '.php';
    public function setLibPath($path, $autoload = false) {
        $this->libPath = trim(trim($path), DIRECTORY_SEPARATOR);
        set_include_path($this->getLibPath());
        if ($autoload) {
            spl_autoload_register(array('Application', 'load'));
        }
        return $this;
    }
    public static function instance() {
        if (!self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }
    //Get the file suffix
    public function getPhpExt() {
        return $this->phpExt;
    }
    //Setting File Suffixes
    public function setPhpExt($ext) {
        $this->phpExt = $ext;
        return $this;
    }
    //Setting the root path
    public function setPath($path) {
        $this->path = rtrim(trim($path), DIRECTORY_SEPARATOR);
        return $this;
    }
    //Setting the path of automatic loading
    public function getLibPath() {
        return $this->path . DIRECTORY_SEPARATOR . $this->libPath;
    }
    //Automatic Loading Function
    public static function load($class) {
        $pos = strrpos($class, '\\');
        if ($pos !== false) {
            $ns = str_replace('\\', DIRECTORY_SEPARATOR, substr($class, 0, $pos + 1));
            $className = substr($class, $pos + 1);
        } else {
            $ns = DIRECTORY_SEPARATOR;
            $className = $class;
        }
        if (strpos($className, '_') !== false) {
            $className = str_replace('_', DIRECTORY_SEPARATOR, $className);
        }        
        $ins = self::instance();
        $classFile = $ins->getLibPath() . $ns . $className . $ins->getPhpExt();
        if (!(include $classFile)) {
            throw new Exception('load class failed: class=' . $class . ' file=' . $classFile);
        }
    }
}
Application::instance()->setPath(dirname(__FILE__))->setLibPath(DIRECTORY_SEPARATOR, true);
//test,Create in the root directory app Catalog,Next, create these two files
new app_user();
new \app\admin();

user.php under app directory

<?php
class app_user{
    public function __construct(){
        new \app\admin();
    }
}

admin.php in app directory

<?php
namespace app;
class admin{}